There are three concerns: 1) terrorism; 2) export/import limitations; and 3) the fire hazard from the lithium contained in the cell phone and laptop batteries.
Terrorism: the difficulty of addressing terrorism is that the level of screening and scrutiny is often arbitrary depending on the current political climate and the capriciousness of the security agent screening your baggage.
Export/import limitations: In most cases, you need to prove that you are carrying the devices for your own personal use and not for resale. As long as the devices are unboxed and in 'used' condition, this shouldn't be difficult.
Fire hazard: I'll mainly address the latter concern regarding the FAA rules on fire hazard.
FAA regulations do not limit the quantity of small lithium cells/batteries carried on board by passengers although they do limit the maximum size (capacity) of larger cells/batteries - such as those used in power wheelchairs and high-powered camera equipment (studio flashes).
However, the batteries must be enclosed in the devices and the devices must be for personal use (and not, for example, for resale) and so you might have trouble convincing the pilot/TSA/flight attendants that you have (say) ten phones for truly personal use.
But five cellphones is actually pretty reasonable by international standards as you might need to swap sims between different phones as you travel between different countries.
49 CFR 175.10(a)(18)
(A) The battery must be removed from the wheelchair or other mobility
aid according to instructions provided by the wheelchair or other
mobility aid owner or its manufacturer;
(B) The battery must be carried in carry-on baggage only;
(C) Battery terminals must be protected from short circuits (by
placement in original retail packaging or otherwise insulating the
terminal e.g. by taping over exposed terminals or placing each battery
in a separate plastic bag or protective pouch);
(D) The battery must not exceed 300 Watt-hour (Wh); and
(E) A maximum of one spare battery not exceeding 300 Wh or two spares
not exceeding 160 Wh each may be carried;
(vi) The pilot-in-command is advised either orally or in writing,
prior to departure, as to the location of the lithium ion battery or
batteries aboard the aircraft.
(18) Except as provided in §173.21 of this subchapter, portable
electronic devices (e.g., watches, calculating machines, cameras,
cellular phones, laptop and notebook computers, camcorders, medical
devices etc.) containing dry cells or dry batteries (including lithium
cells or batteries) and spare dry cells or batteries for these
devices, when carried by passengers or crew members for personal use.
Portable electronic devices powered by lithium batteries may be
carried in either checked or carry-on baggage. Spare lithium batteries
must be carried in carry-on baggage only. Each installed or spare
lithium battery must be of a type proven to meet the requirements of
each test in the UN Manual of Tests and Criteria, Part III,
Sub-section 38.3 and each spare lithium battery must be individually
protected so as to prevent short circuits (e.g., by placement in
original retail packaging, by otherwise insulating terminals by taping
over exposed terminals, or placing each battery in a separate plastic
bag or protective pouch). In addition, each installed or spare lithium
battery must not exceed the following:
(i) For a lithium metal battery, a lithium content of not more than 2
grams per battery; or
(ii) For a lithium ion battery, the Watt-hour rating must not exceed
100 Wh. With the approval of the operator, portable electronic devices
may contain lithium ion batteries exceeding 100 Wh, but not exceeding
160 Wh and no more than two individually protected lithium ion
batteries each exceeding 100 Wh, but not exceeding 160 Wh, may be
carried per person as spare batteries in carry-on baggage.
(iii) For a non-spillable battery, the battery and equipment must
conform to §173.159a(d). Each battery must not exceed a voltage
greater than 12 volts and a watt-hour rating of not more than 100 Wh.
No more than two individually protected spare batteries may be
carried. Such equipment and spare batteries must be carried in checked
or carry-on baggage.
(iv) Articles containing lithium metal or lithium ion cells or
batteries the primary purpose of which is to provide power to another
device must be carried as spare batteries in accordance with the
provisions of this paragraph.
(19) Except as provided in §173.21 of this subchapter, battery-powered
portable electronic smoking devices (e.g., e-cigarettes, e-cigs,
e-cigars, e-pipes, e-hookahs, personal vaporizers, electronic nicotine
delivery systems) when carried by passengers or crewmembers for
personal use must be carried on one's person or in carry-on baggage
only. Spare lithium batteries must be individually protected so as to
prevent short circuits (by placement in original retail packaging or
by otherwise insulating terminals, e.g., by taping over exposed
terminals or placing each battery in a separate plastic bag or
protective pouch). Each lithium battery must be of a type which meets
the requirements of each test in the UN Manual of Tests and Criteria,
Part III, Sub-section 38.3. Recharging of the devices and/or the
batteries on board the aircraft is not permitted. Each battery must
not exceed the following:
(i) For lithium metal batteries, a lithium content of 2 grams; or
(ii) For lithium ion batteries, a Watt-hour rating of 100 Wh.
The IATA has similar regulations with similar restrictions:
184.108.40.206 Portable Electronic Devices (including medical devices) containing Batteries
220.127.116.11.1 Portable electronic devices (including medical devices) (such as watches, calculating machines, cameras, cellular phones,
lap-top computers, camcorders, etc.) containing batteries when carried
by passengers or crew for personal use, which should
be carried in carry-on baggage.
(APCS/Cargo 15/12/2014 IATA Lithium Battery Guidance Document - 2014 Page 12)