[cisco-voip] SIP Trunk providers

[cisco.voip]

I am tring to setup a small office with a cisco UC500 for their phone 
system.
I want to connect the calls over the internet connection via a SIP provider 
and use the FXO port for SRST backup.
 Does anyone have a recommendation for a SIP provider to terminate internet 
calls to PSTN for a reasonable price.
Also, how do I manage the pone number for dialing access, if the local pstn 
line is number 301.123.4567, how do I get that number to be serviced
by the sip provider.

Thanks