Infrastructure-based mobile networks are becoming increasingly overloaded due to strong growing number of mobile devices like smartphones and their communication needs. Especially in urban cities the cost of maintaining and extending infrastructure is high due to increased geographical density of mobile devices. While this increasing density puts high load on infrastructure-based networks, it is an enabler for infrastructure-less networks like Delay Tolerant Networks that perform end-to-end routing through store-carry-forward. In this work we present a novel routing scheme for offloading traffic from infrastructure-based networks with the help of Delay Tolerant Networks. Messages are initially routed in the infrastructure-less network, and continuously switch over to infrastructure-based routing when the probability of successful delivery in the infrastructure-less network decreases. We analyze the scheme under different types of heterogeneity by varying the fraction of infrastructure-capable devices, fraction of DTN-capable devices, and message size. Our scheme allows to offload larger parts of traffic from infrastructure networks. For example, using a message time-to-live of 5 h our scheme can offload 36% of traffic from infrastructure networks with acceptable ad-hoc forwarding load and storage requirements on mobile devices.