Codecs are another fun detail. Basically you get g711 with ulaw and alaw flavors where ulaw is the US and alaw is everyone else with any system which is uncompressed PCM. It is good for networks with some packet loss and/or some latency but uses 64kb/s for data and then another 30 or so in protocal overhead. You can use something like iLBC or GSM for free also which cuts the data requirements down to 13-14kb/s but still has 20-30kb/s in SIP protocal overhead.
Faxing only works over g711 only as it is non-compressed. any compression codec nukes faxes basically because a lost packet causes a 'fill' where the gap in audio you might here is replaced by replaying the previous sound. There is t38 faxing which helps but just stick with g711 for faxing.
I like to use ulaw for any and all local calls. I also like ulaw to my SIP provider when I have the bandwidth available. With remote workers I usually do iLBC because they are likely to be using a softphone on their laptops and using an aircard which likely to drop packets so I need low bandwidth and resiliency to packet loss which iLBC handles pretty well. GSM is also decent. You could hear a little bit of interfearance but this is typically acceptable.
For phones I am a die-hard Aastra fan. I like their 55i and 57i phones a lot and deploy them almost exclusively.
www.voipsupply.com/aastra-55i www.voipsupply.com/aastra-57i
I should add that there are a lot of good phones out there like Polycom (great audio quality) and Snom. Granstream makes some very nicely featured phones but the build quality of their phones is reflected in the price. I like Aastra phones because they have an XML browser on them and you can do things like visual voicemail, company directory, google search, weather, etc etc.. They also can be provisioned very very easily as you can log into a phone on an established extension without having to program the phone individually.
For interface cards, go with Sangoma. They are the best. Best build, best sound. Rhinos are good also but dont sound as good. I dont like digiums cards much as I have had problems with multiple cards in one box. generics are garbage. I would do a grandstream as an FXO before going with a generic hardware card.
you may also notice my vendor of choice is voipsupply. Their sales rep Arthur has been great and their prices very good.
www.voipsupply.com/