[SR-Users] How do I make subst_hf() use variables in search (not replacement)?

Consider the following snippet: if (is_present_hf("Contact")) { xlog("L_ALERT", "===== reply to SUBSCRIBE has Contact: $ct\n"); xlog("L_ALERT", "===== want to replace with $td\n"); xlog("L_ALERT", "===== regexp to use is /$sel(cfg_get.asterisk.bindip):$sel(cfg_get.asterisk.bindport)/$td/\n"); if (subst_hf("Contact", "/127.0.0.1:5080/$td/", "a")) { xlog("L_ALERT", "===== reply had Contact modified\n"); } } If I use the hardcoded regexp "/127.0.0.1:5080/$td/", subst_hf() replaces the Contact value correctly. However, if I use "/$sel(cfg_get.asterisk.bindip):$sel(cfg_get.asterisk.bindport)/$td/" (where asterisk.bindip and asterisk.bindport are the supposed values 127.0.0.1 and 5080), subst_hf() fails to replace the Contact value. How do I make use of the existing configuration variables in order to substitute in the Contact header?