To add to Ajay's correct posting - even if you had an external router to handle the BGP and the agreement from the provider who has assigned you your /28, you could not use the first provider's assigned netblock with a second provider since it is smaller than a /24 - the smallest allowed portable block.
If you were to acquire a provider-independent /24 assigned directly to you from your provider or RIR (ARIN, RIPE, APNIC, etc.) you could potentially use that (still requiring an external router). However it is unlikely that you could get a new one assigned since the /28 suffices for your hosts at this point.
As you describe it now, if you lose a provider, you will lose all your sessions, even under best practices and best possible configuration scenario.
One alternative may be to inquire about redundant service from your preferred provider. Depending on your location's service options, this may be possible. You would be to be diligent in ensuring that the redundancy is adequate to protect your connectivity (e.g., different upstream devices, diverse connections into your building, etc.) In such a service scenario, your provider may be able to retain your IP space and managing routing to you dynamically.
Hope this helps.
