To match the local number with the external number during a SRST failover, you can use the command dialplan-pattern <<tag_pattern>> extension-length <<length>>. this command also ensures that a full E.164 address is sent as the ANI for outgoing calls. The dialplan-pattern command causes the SRST router to create an additional virtual dial peer for local extension corresponding to the pattern specified in the command.So, this command can be used to map the DNIS sent by the PSTN to a local extension.

Pay attention that if you play with translation rules on your voice-port, it may be a good choice also to continue with digit manipulation to conform your SRST dialplan as the translation rule are processed before.