The Kenyan telephone system is still largely based on copper wires and still uses some outdated equipment. Over the last years, Telkom Kenya has replaced most older equipment and they are still doing so. You'll find communications from larger towns quite OK, but in rural area's land lines can be iffy, if not unusable.
Mobile phones are now widely used. If you are new in Kenya, take your GSM phone along and buy a pre-paid SIM card from one of the two (soon three) providers. They are cheap and airtime (e.g. on scratch cards) can be purchased almost everywhere. Coverage within Western Kenya is almost 100% (consult the website of your provider). SMS to numbers abroad usually works OK.
For international voice communications the two mobile providers as well as Telkom Kenya now have introduced a VoIP service. Dial 888 instead of the normal international exit code (000) followed by the country code and the rest of the number and you'll be using their VoIP system. Quality is good and the costs are much lower than a normal landline. Using 888 on a mobile is more expensive than on a land line.