I have this exact setup using VoIP on a mini Asterisk server. A free UK DID (London 0207 in my case) supplied by Sipgate and bulk termination to UM's numbering range ranging in price from US$0.07 (a fluke but it still works) to $0.22/minute gives you reliable service.
For mini Asterisk setups I would go the SBC way (single board computer). It can't handle transcoding, but it does redirect just fine. A good example of such an SBC is WRAP from PCengines (
www.pcengines.ch).
VladS
Mobile phones: iPhone 5, Blackberry 9900, Nexus S, Samsung S3322 duos
Mobile data cards: Huawei E587u-5, Huawei E583c, Huawei E160
Postpaid SIMs: CA: Fido, Wind; INTL: Telna
Prepaid SIMs: DE: Fonic, Lidl; AT: yesss!, bob; UK: O2; US: AT&T; RO: Orange, Vodafone; FR: b&you, Lycamobile; NL: Lycamobile; BE: Lycamobile, Jim Mobile; CL: Entel; MX: Telcel; INTL: eKit Blue, eKit Yellow
Dead SIMs: too many to list